Every so often, just when he's least expecting it, a cheque for 29p drops onto Mike Leigh's doormat courtesy of Mel Smith and Griff Rhys Jones. It's around 20 years now since the duo performed a sketch entitled A Mike Leigh Play in their television series Alas Smith and Jones. Back then, Leigh's agent negotiated this princely sum, and now, when the show gets flogged to, say, Indonesia, there's another 29p in the kitty. Slumped in an armchair in his Soho office, wearing his customary ensemble of beiges, browns and greys, the 62-year-old writer-director can still recall the sketch. "It had all these people sitting around, grunting. Finally they fell face down into the corn flakes." There is not much sign of mirth from behind his heavy beard. Goodness knows what they make of the sketch in Indonesia.

This domestic dourness was once the abiding impression of Leigh's work, and he thinks it's what people mean when they refer to something being "like a scene from a Mike Leigh film". He appreciates being part of the language in this way, but believes that he stopped making that kind of movie after Life Is Sweet in 1990. Certainly, Smith and Jones's disparagement will mean little to those who have seen Two Thousand Years, Leigh's first play in over a decade. It sold out long before anyone knew that it concerned the tribulations of a liberal Jewish family that finds it has religious orthodoxy in its midst; in fact, it sold out long before it had a title.

At a screening in the late 1990s, Leigh was asked if, being Jewish himself, he would ever write about a Jewish family. "It's a possibility," he replied reluctantly. "And you can't get a more Jewish answer than that." He tells me now that Two Thousand Years has been gestating for some time, though he won't discuss the play: it is, after all, a living entity. He pops over to the National every 10 days to keep an eye on it. "It helps the actors. Plays are prone to change. And I get a buzz out of it." What he will go into, at enthusiastic length, is the subject of a new retrospective of his work. "I like my films," he says, beaming. "I can't understand directors who don't watch their films after they've made them. If you don't like them, how the fuck can you expect anyone else to?" He's pleased that work he considers neglected - he singles out Home Sweet Home, Four Days in July and Career Girls, to which I would add the brilliant Kiss of Death - will be screened again, and mourns only the film that can't be found: Knock for Knock, which he made in 1976, was wiped in order to save space at the BBC. "If I found out who made that decision, I'd go and shoot them. Without batting an eyelid."

The season charts the rocky course from Leigh's 1971 debut Bleak Moments through what he calls "the whingeing wilderness years when I just wanted to make another [feature] film". During those 17 years, television films like Nuts in May, Abigail's Party and Meantime were cultural landmarks that attracted millions of viewers. Still, it was 1988 before he made High Hopes, his second film for the cinema. "If someone had said it would take that long, I would have jumped off Waterloo bridge." Since High Hopes, his movies have taken him in unexpected directions, to increasing acclaim: Naked, Secrets and Lies, Topsy-Turvy, Vera Drake.

The commercial and critical success of Two Thousand Years, combined with the retrospective next door at the National Film Theatre, marks the culmination of a fruitful period. After the inexplicable snubbing of Vera Drake by Cannes, that film became a magnet for awards and approbation, landing five Oscar nominations including best director. Incredibly, the honour has done nothing to convince financiers that Leigh is a horse worth backing. "We're battling to get money for the next film," he sighs. "I'm sure we'll get it, but it's tough."

As it was back in 1970, when Leigh was a 27-year-old cobbling together the cash to turn his tenth play Bleak Moments into a film. Albert Finney eventually stumped up most of the £14,000 budget, and donated some unused film stock. Even today, this story of a loveless typist, her repressed suitor and her disabled sister looks boldly experimental. Scenes are agonizingly protracted and played so quietly that you can hear the characters' expectations drop. One harrowing passage, detailing a wretched date in a near-deserted Chinese restaurant, could be screened by unscrupulous regimes to extract confessions from dissidents.

"The film rivets audiences," says Leigh. "I was at a university screening in the 1970s. It was midnight, there were lots of pints on the go. And when it finally got to that moment - the nearest thing the film has to a climax - when Anne Raitt says to Eric Allan, 'I was thinking it would be great if you took your trousers off' ... well, the audience let out the most almighty cheer." The picture's challenging style, along with the improvisatory techniques employed to create all Leigh's work, had its roots in the director's exposure to world cinema when he arrived in London from Manchester in 1960. "There was so much going on: Cassavetes, Shirley Clarke, the nouvelle vague. This whole idea of getting out onto the streets and making a film, rather than adhering to studio-bound conventions, got me thinking about working organically to create something. As a kid going to the pictures, I'd always had this thing about putting on the screen people as they really are, as opposed to actors in a movie."

Leigh has described his working methods so many times that he must feel like tattooing his words onto the forehead of anyone who asks him for an explanation. For the uninitiated, though, it's important to know that when he begins auditioning actors, he has only vague ideas about the nature of what lies ahead; the material is conceived and honed by Leigh and his cast over months of improvisation. Even in projects where the territory is predetermined - such as Four Days in July, set in Belfast, or Topsy-Turvy, which drew for the first time in Leigh's career on factual material - nothing is fixed until just before the cameras roll. "We had shot four-fifths of Secrets and Lies before we did the massive improvisation that yielded up the barbecue scene at the end. Similarly, I couldn't have told you the ending of Naked until two or three days before we shot it because I couldn't work out what it was going to be. Then I was driving to the location one morning and I thought: 'Fuck!' Or, put another way, 'Eureka!'"

Any actor chosen to work with Leigh will have a fair idea of the process, which is one sign of how things have changed since the early days. "I remember sitting on the phone to agents and saying endlessly: 'No, there's no script, you see, what we do is ...' Eventually they'd say, 'My client is not getting involved with this.' Now it takes me months to get through a tiny slice of the people who want to audition."

The roll-call of actors who have passed through Leigh's hands is so illustrious that you feel you should stand up and sing the national anthem whenever someone runs through it: Brenda Blethyn, Jim Broadbent, Katrin Cartlidge, Phil Daniels, Philip Davis, Shirley Henderson, Lesley Manville, Timothy Spall, Imelda Staunton, Alison Steadman (Leigh's ex-wife), David Thewlis. Many have proceeded to international success: Andy Serkis, who spent so long hanging out with futures traders to research his role in Career Girls that they offered him a job, is better known now as Gollum in the Lord of the Rings movies; Marianne Jean-Baptiste, so moving in Secrets and Lies, brings a touch of class to the hit US series Without a Trace. Gary Oldman and Tim Roth, who appeared together in Meantime, haven't done badly either. The NFT season throws up other forgotten treats - Ben Kingsley as a kindly mini-cab driver arranging an abortion in Hard Labour, or Lindsay Duncan, playing against the elegant roles that would become her lot, as a dowdy schoolteacher in Grown-Ups.

Is Leigh aware that some of the actors who started out with him now say that his intensive methods "spoiled" them - that everything which followed was a come-down? "Of course I'm aware! I can see it as clear as I can see this table." He is booming now, and he has just slapped the coffee table. "I do have mixed feelings about it. I don't want to screw people up." Then there are those perfectly good actors that he has never got round to working with a second time. "A few of them get quite upset. They think, 'He doesn't like me.'" Some aren't even on Leigh's Christmas card list. "You could count them on one hand. They delivered the goods but didn't know how to behave. And I wouldn't dream of working with them again." Naturally, my mind races scurrilously through all the people who have appeared in just one Mike Leigh film.

Leigh believes that the amount of actors who prize working with him undermines the accusation that he pities or patronises his characters. He didn't bring up this enduring criticism - I did. Facing it yet again, Leigh sinks in his chair. The energy visibly leaves him. He makes eye contact less readily, and gazes out of the window. "It's just off the mark. It's so irrelevant that I don't take it seriously."

I was trying to think of those charges against him that won't go away. No one questions any more that Leigh is a "real" writer, as they sometimes used to when they discovered how fully his actors participated in the creative process. He thinks the confusion was his fault - until 1987, his credit was always "devised and directed by". Then he switched it to "written and directed by". "It should always have been that. Nothing you can do about it now."

But there have always been concerns about Leigh's attitude toward his characters, from the moment the shrill, social-climbing Beverly in Abigail's Party clutched a bottle of beaujolais and promised to "just pop it in the fridge". I think the popularity of television comedies influenced by Leigh - The Royle Family, The Office, Early Doors, Shameless, Phoenix Nights - will leave newcomers to his work better prepared for the cringe-making behaviour of Gloria, the downhearted banshee in Grown-Ups, or the obsessive autograph-hunting clerk Giles in Who's Who. But other creations are less easily reconciled. Leigh must have noticed, I insist, that many viewers can't swallow the champagne-swigging, Porsche-zooming yuppie rapist in Naked. "Well, he's my most unsympathetic character by a long chalk." I believe it's more to do with people not believing in the character than not liking him: that he doesn't fit this otherwise gritty film. "That's a matter of perception. Which is fair enough. But I don't agree. And we've given far too much time to a question that's irrelevant. Though I'm sure it will dominate your article."

In his flashes of distaste or enthusiasm, Leigh reveals himself most intimately. It's a bit of a slog asking him anything about his life. When he mentions in passing that his partner, Charlotte Holdich, designed the costumes for A Very Social Secretary, I feel disproportionately grateful for this meagre crumb of information. "What else do you want to know?" he says in a playfully gruff voice that indicates he'd rather not tell me anything.

Well - do you play golf? "I do not play golf," he fumes. "Nor am I ever likely to. What else?" Can you knock together a decent meal? "I am a famously good cook. Next."

Time for the big one: have you found the secret of happiness? "Yes. Next."

What's the secret, then? "Now you're just trying to end the interview." He rises from his chair. "If you want to find out anything else, you can just go home and Google me"

· The Mike Leigh season runs until 30 November at the National Film Theatre, SE1. Details: 020-7928 3232. Two Thousand Years is in rep at the National Theatre, London SE1 (020-7452 3000).